Britain's newest art venue has none of the trappings you usually expect. There are no sheets of planar glass and no crisp white plaster and stainless steel handrails. Its duties of creating the right environmental conditions for artworks are discharged by humidifiers freestanding on the floor, not expensively integrated into the structure. It will open in two weeks with minimal fanfare. It has received no public money and is credited to no architect, at least none since John Loughborough Pearson, who died in 1897. The White Cube it is not.

This is Two Temple Place, once Astor House, a late Victorian building in the style of a miniaturised Tudor mansion. Outside, it is stony and respectable. Inside, it is one of those secret universes, lavish, ornate and slightly crazed, in which London specialises. In its various incarnations, as the Incorporated Accountants' Hall, as the offices of the medical devices company Smith & Nephew, and as a venue for corporate parties, it has never been open to the casual visitor. Now, like Leighton House or Sir John Soane's museum, it is being added to the list of visitable internal curiosities.

Facing the Thames, a little upstream from Somerset House, it was built by the fabulously wealthy 1st Viscount Astor, who emigrated to Britain from America because of fears that his children would be kidnapped. He seems to have suffered from the plutocratic paranoia that now has iris-recognition systems and SAS-trained security at One Hyde Park. One of his homes, Hever Castle, had a drawbridge that was raised every night.

Astor House was built as the office from which he ran his estates. Compressed within its walls is the cultural booty of a man who felt able and entitled to appropriate whatever he wanted. Its mahogany and oak stair rises from a floor, similar to one in Westminster Abbey, in marble, jasper, porphyry and onyx, up towards a gallery ringed by columns of solid ebony. It is encrusted with carving and statues taken randomly from literature, from The Three Musketeers, The Last of the Mohicans, Othello, The Famous History of the Life of King Henry VIII.

Off the gallery is Astor's study, a nest of decorated wood, and a great hall, where he used to conduct his business meetings, with a high, hammer-beam roof in Spanish mahogany, a commanding river view and outbreaks of gilt and bronze. It is panelled in pencil cedar, chosen for its fragrance as well as its grain. And there is more eclectic iconography. There are Pocahontas, Diane de Poitiers, Machiavelli, Ignatius Loyola, Bismarck and a clutch of decapitated queens: Anne Boleyn, Marie Antoinette and Mary, Queen of Scots. There are nine Arthurian heroines, all looking comely in much the same way. At the east and west ends, large stained-glass windows depict Swiss landscapes at sunrise and sunset.

Big beasts of history and art are captured and mounted as a game hunter might gazelle heads and tiger rugs. Rare timbers and stones are extracted from distant places, and mastered with exacting craftsmanship. It is an extraordinary statement of power, ownership and wealth, although also of dislocation. Astor's greediness for culture feels like an over-eager attempt to root himself. The weird choices put him everywhere and nowhere. Outside, the Tudor manor sits oddly among the office blocks in what property promoters now call "Midtown", the borderlands of the cities of London and Westminster.

Now, thanks to a newer generation of wealth, it is opening up. Richard Hoare, of a leading banking family, bought it for the charitable foundation the Bulldog Trust, so it could earn revenue from hiring it out. But a public use was also sought and it eventually decided to make it a London showcase for the collections of regional museums. The trust is starting, somewhat cautiously, with works from the William Morris gallery in the outer London district of Walthamstow which, if not technically "regional", evidently feels like it from the perspective of the city's centre.

Students enrol at the University of the Arts London. Photograph: Frantzesco Kangaris for the Observer

The University of the Arts London, the £200m new campus for six London art colleges including Central Saint Martins College of Art and Design, uses the more usual repertoire of contemporary cultural architecture – glass, concrete, plywood and the distressed brickwork of re-purposed industrial relics, rather than onyx, ebony and fragrant cedar. It consists of the 1852 Granary Building, a brick cuboid that was once the organ by which grain from the north was gulped into London, and a new block, long and oblong, behind it.

It stands at the centre of the long-awaited, still-ongoing redevelopment of King's Cross and it is a work of enlightened self-interest. From the developer's point of view, it is a clever way of animating the site, to fill the place with thousands of arts and design students, whose energy, stylishness and looks are somewhat above the national average. From the colleges' point of view, it allows all their departments to be placed under one roof.

The architects Stanton Williams have decided to make a robust, straightforward building that can take the battering art students will give it, but also be a background for whatever they want to put on there – exhibitions, catwalk shows, installation, performance. Down the middle runs a central covered "street", broad and bright, which is intended as a social centre, animated with the help of galleries and stairs. The architects hold back from expressing themselves too much, leaving that to the students.

As of last week the students had not yet taken full command of the building and a highly unscientific poll revealed instead some grumbles about insufficient space, but it is early days. It is possible also that the design is too aloof: greater use of bashable stuff like timber would have been nice, instead of corporate glass, and the building has dead-straight galleries which could usefully have warped and bulged in places, to vary the rhythm of use.

But to do too little with such a place is better than doing too much and it gets the main moves right. It is powerful and direct, in the spirit of the old industrial buildings, but also habitable. To have so much aspiration and making going on in one place is phenomenal and the architecture allows this phenomenon to do its stuff and speak for itself.